The present invention relates generally to switched-mode power converters with circuitry controlling synchronous rectification. More particularly, the present invention relates to circuits for controlling the operation of synchronous rectifiers during a startup condition into pre-biased output voltage such that reverse current flow is reduced or eliminated.
DC-DC power converter devices are currently applied in numerous applications of power systems. These converters are electronic devices that convert a direct current (DC) input voltage into a predetermined, or nominal, DC output voltage. A typical isolated DC-DC converter includes a transformer with primary and secondary windings about a common magnetic core. A typical non-isolated converter does not require a transformer, but may instead comprise a simple DC input terminal. In either case, one or more power switches are provided that may be opened and closed to control energy transfer. In various examples of DC-DC converters, such as 5V or 12V output converters previously in common use, free-wheeling diodes were used to prevent reverse current flow from the converter output into the switching circuitry when a switch was closed. However, the use of diodes to perform this function resulted in significant inefficiencies such as power loss. These inefficiencies are even more significant in a relative sense for low output voltage converters and power supplies.
Synchronous rectification has become a desirable alternative to free-wheeling diodes in power converters due to the increased power conversion efficiency that results at least in part from reduced power losses and higher density. However, while free-wheeling diodes only permit current flow in one direction, synchronous rectifiers such as bipolar transistors, MOSFETs, or other equivalent semiconductor switches permit current flow in either direction. As a result, such converters may have both current-sourcing and current-sinking functions.
A problem common to many output power stages of power supplies employing synchronous rectification is therefore the drawing of current from a pre-existing voltage, or pre-biased output voltage, also known as reverse bias or back bias, during certain sequences such as startup or shutdown conditions. Pre-biased voltage may come from other power sources in a non-isolated system, or may come from a load. During a soft-start condition, the synchronous rectifiers may have a high duty ratio for the duration of the output voltage rise time of the power supply. Where a pre-biased voltage exists, a negative current may then be built in the filtering inductor, which may cause the output voltage to drop and correspondingly disturb other elements in the system.
FIG. 1 shows a typical prior art circuit for providing bias voltage to the synchronous rectifier driver 18 in a non-isolated converter. Q1 in this example represents a control rectifier while Q2 represents a synchronous rectifier.
FIG. 2 shows a typical prior art circuit for providing bias voltage to the synchronous rectifier driver 18 in an isolated converter. Q1 and Q2 in this example both represent synchronous rectifiers.
In either circuit as shown, the output voltage is measured across a filtering LC circuit and fed back to an error amplifier 14. The error amplifier 14 compares the feedback voltage to a reference voltage. During normal operation the error amplifier 14 may then produce no further signals, but during predetermined conditions such as a system startup for example the error amplifier 14 may instead provide a signal to the pulse width modulator (PWM) controller 16, which supplies a series of pulse signals to the driver 18 indicative of the condition. The driver 18 then provides control signals to rectifiers Q1, Q2 based on the pulse signals from the PWM controller 16 and having an amplitude associated with a driver supply input voltage V_driver.
However, immediately enabling the rectifiers Q1, Q2 in the presence of pre-biased voltage leads to reverse current flow of the inductor current. During the output voltage rise time period that is inherent to the specific power device, the reverse current flow is a significant problem as described above. Some prior art circuits have addressed this problem with circuits and methods that are undesirable for certain applications because they require additional circuitry for detecting system parameters or voltage clamping. These are generally impracticable where less space is physically available on a circuit board, and also cause additional problems due to increased numbers of components and cost.
Therefore, it is desirable that a circuit be provided for controlling synchronous rectifiers in a manner that reduces or eliminates reverse bias current draw.
It is further desirable that a circuit be provided for providing the above capabilities during a predetermined condition such as a soft start period for a power converter.
It is further desirable that the circuit efficiently and cost-effectively provide the above capabilities, with the ability to design for a broad range of foreseeable applications and outputs.